Correct a Psych::DisallowedClass error in tests

After upgrading to Ruby 3.1, following error raises when running
redmine_git_hosting tests depending on loading a fixture based on a YAML
file:

    Psych::DisallowedClass:
           Tried to load unspecified class: Time

Author: dosyfier

spec/support/global_helpers.rb

@@ -84,7 +84,8 @@ def create_svn_repository(**opts)
   end
 
   def load_yaml_fixture(fixture)
-    YAML.load load_fixture(fixture) # rubocop: disable Security/YAMLLoad
+      YAML.load load_fixture(fixture), # rubocop: disable Security/YAMLLoad
+        permitted_classes: [Symbol, Time] if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('3.1.0')
   end
 
   def load_fixture(fixture)